Defrost controls for heat pumps



W. K. KYLE Sept. 16, 1969 DEFROST CONTROLS FOR HEAT PUMPS 2 Sheets-Sheet 1. COMPRESSOR C Filed May 15, 1968 IFM SOLENOID RVS THERMISTER OUTDOOR COIL HERMISTER CAPILLARY TUBE CMSS II If OFM OFITSS ll OFMS DRSI KYCMS V RS2 AL KQVS D \J RECTIFIER RB MPLBFIER A THI RI 23 DR33 INVENTOR WILLHAM K.KYLE BY W QM ATTORNEY United States Patent M 3,466,888 DEFROST CONTROLS FOR HEAT PUMPS William K. Kyle, Staunton, Va., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed May 15, 1968, Ser. No. 729,364 Int. Cl. F2513 13/00 US. Cl. 62-156 7 Claims ABSTRACT OF THE DISCLOSURE For controlling the defrosting of the outdoor coil of a head pump when the outdoor coil is operating as an evaporator coil, and the indoor coil of the heat pump is operating as a condenser coil for heating indoor air, a first thermistor is in heat exchange contact with the outdoor coil, and a second thermistor is exposed to the temperature of the outdoor air. The two thermistors are connected in series. When frost forms on the surface of the outdoor coil, the temperature of the refrigerant therein decreases, and the voltage at the junction of the thermistors changes, and operates a control circuit which stops the fan of the outdoor coil, and reverses the flow of refrigerant so that the outdoor coil operates as a condenser coil to melt the frost.

Field of the invention The field of the invention is heat pumps used to cool or heat indoor air. When a heat pump is operated to heat indoor air, its reversal valve is positioned to route refrigerant gas from the compressor of the heat pump through the indoor coil of the heat pump, and through expansion means into the outdoor coil of the heat pump. At low outdoor temperatures, frost may form on the surface of the outdoor coil, insulating it so that it cannot effectively absorb heat from the outdoor air. It is usual, as disclosed in my (joint with R. S. Stewart) US. Patents Nos. 3,103,793 and 3,103,794, to use an air pressurestat which, when there is a predetermined pressure drop in the air passing over the surface of the outdoor coil, caused by frost, energizes a defrost relay which opens switches to stop the motor of the fan of the outdoor coil, and to deenergize the solenoid of the reversal valve, following which the latter routes gas from the compressor through the outdoor coil to operate the latter as a condenser coil for melting the frost. A pressurestat responsive to the increase in the pressure of the refrigerant within the outdoor coil caused by the melting of the frost, or a thermostat responsive to the increase of the temperature of the refrigerant within the the outdoor coil caused by the melting of the frost, deenergizes the defrost relay, resorting the heat pump to heating operation.

This invention replaces such an pressurestat, and such a refrigerant pressurestat (or thermostat) with a pair of relatively inexpensive thermistors. In one embodiment of this invention, such a defrost relay and its switches are replaced by a pair of relatively inexpensive transistors.

Summary of the invention A heat pump has a refrigerant compressor, an outdoor coil, an indoor coil, a reversal valve, and expansion means, During air cooling operation, the reversal valve is positioned to route discharge gas from the compressor through the outdoor coil operating a condenser coil, and the expansion means into the indoor coil operating as an evaporator coil. During air heating operation, the reversal valve is positioned to route discharge gas from the compressor through the indoor coil operating as a condenser coil, and the expansion means into the outdoor coil operating as an evaporator coil. The reversal valve has a sole- Patented Sept. 16, 1969 noid which, when deenergized, places the reversal valve in its cooling position, and, when energized, places the reversal valve in its heating position. A first thermistor is in heat exchange contact with the outdoor coil. A second thermistor is exposed to the temperature of the ambient outdoor air. The two thermistors are connected in series in one leg of a bridge, in the other leg of which are two series connected resistors. When frost forms on the surface of the outdoor coil while it is operating as an evaporator coil, it acts as insulation so that the absorption of heat from the outdoor air is greatly reduced. As a result, the temperature of the refrigerant within the outdoor coil decreases; the temperature of the thermistor in heat exchange contact with the outdoor coil decreases; the voltage at the junction of the thermistors increases; the bridge is unbalanced, and supplies current to energize a defrost relay, directly if the latter is a sensitive, polarized relay, or through a conventional amplifier. A normally closed switch of the relay opens, and deenergizes the motor of the fan of the outdoor coil. Another normally closed switch of the relay opens and deenergizes the reversal valve solenoid which converts the heat pump to cooling operation so that the outdoor coil operates as a condenser coil and melts the frost. For preventing hunting when the thermistor in contact with the outdoor coil starts to warm when the defrosting starts, another, normally open switch of the relay closes, and shorts out a portion of one of the resistors of the bridge, thereby increasing the voltage at the junction of the thermistors.

In another embodiment of the invention, a pair of relatively inexpensive transistors replace the defrost relay and its switches.

Brief description of the drawings FIG. 1 is a diagrammatic view of a heat pump embodying this invention;

FIGS. 2a, 2b, 2c, and 2d are diagrammatic views of the starter of the compressor motor, of the starter of the outdoor fan motor, of the defrost relay when used, and of the heat relay when used, respectively, of the heat P p;

FIG. 3 is a simplified circuit schematic of one embodiment of the control system of the heat pump, and

FIG. 4 is a simplified circuit schematic of another embodiment of the control system of the heat pump.

Description of the preferred embodiment of the invention Referring first to FIG. 1 of the drawings, heremetic refrigerant compressor C, having an enclosed, electric motor CM, is connected by discharge gas tube 10 to reversal valve RV which is connected by tube 11 to outdoor coil OC, and by tube 12 to indoor coil IC. The coils OC and 1C are connected by a capillary tube expansion means 14 containing a restrictor 15 such as is disclosed in the US. Patent No. 2,785,540 of G. L. Biehn. The valve RV is connected by suction gas tube 13 to the suction side of the compressor C. A fan OF driven by an electric motor OFM, moves outdoor air over the surface of the coil OC. A fan IF driven by an electric motor IFM, moves indoor air over the surface of the coil IC. A PTC thermistor TH1 is exposed to the temperature of the outdoor air adjacent to the coil 0C. A similar thermistor TH2 is in heat exchange contact with a surface such as a return bend, of the coil OC.

Referring now to FIGS. 2a, 2b and 2c of the drawings, compressor motor starter CMS has switches CMSS which close when the starter CMS is energized; outdoor fan motor starter OFMS has a switch OFMSS which closes when the starter OFMS is energized, and defrost relay DR has normally closed switches DRSI and DRSZ which open, and has a normally open switch DRS3 which closes when the relay DR is energized.

Referring now to FIG. 3 of the drawings, the compressor motor CM is connected by its starter switches CMSS to the lines L1 and L2 of an AC supply source. The outdoor fan motor OFM is connected by its starter switch OFMS to the lines L1 and L2. The fan motor starter OFMS is connected in series with conventional, cooling control thermostat CT, and the normally closed siwtch DRSI of the defrost relay DR to the lines L1 and L2. The compressor motor starter CMS is connected in series with the thermostat CT, and in series with a conventional, heating control thermostat HT, to the lines L1 and L2. The reversal valve solenoid RVS is connected in series with the thermostat HT and the normally closed switch DRS2 of the defrost relay DR to the lines L1 and L2. The solenoid RVS is normally deenergized, and normally holds the reversal valve RV in its cooling position. A conventional rectifier bridge RB has its input connected in series with the thermostat HT to the lines L1 and L2, and has its output connected to positive bus 20 and to negative bus 21.

The starter of the indoor fan motor IFM is not shown since not involved in the control system. The motor IFM would be started when the compressor motor is started as is usual.

One end of the thermistor TH1 is connected to the negative bus 21, and its other end is connected to one end of the thermistor TH2, the other end of which is connected to the bus 20. A variable resistor R1 is connected at one end to the bus 20, and at its other end to one end of resistor R2. The other end of the resistor R2 is connected to the bus 21. The resistor R2 has a tap 23 connected through the normally open switch DRS3 of the defrost relay DR to the bus 21. The junction of the thermistors TH1 and TH2 is connected to one input connection, and the junction of the resistors R1 and R2 is connected to the other input connection, of a conventional amplifier A, the output of which is connected to the defrost relay DR. The thermistors and the resistors are seen to be connected in a normally balanced bridge circuit, the output of which is connected to the amplifier A. If a sensitive defrost relay is used, the amplifier A can be omitted, and the defrost relay connected directly to the output of the bridge containing the thermistors.

Cooling operation of FIGS. 1-3

The solid line arrows alongside the tubing of FIG. 1 show the direction of refrigerant flow during cooling operation. The reversal valve is in its cooling position since its solenoid RVS is deenergized. When the thermostat CT calls for cooling, it energizes the compressor motor starter CMS which closes its switches CMSS, energizing the compressor motor CM, and energizes through the normally closed switch DRSl of the defrost relay DR, the fan motor starter OFMS which closes its switch OFMSS, energizing the outdoor fan motor OFM.

The compressor C supplies discharge gas through the tube 10, the reversal valve RV, and the tube 11 into the outdoor coil operating as a condenser coil. Liquid condensed within the coil 0C is expanded within the capillary tube 14 and supplied to the indoor coil IC operating as an evaporator coil. Gas flows from the coil IC through the tube 12, the reversal valve RV, and the tube 13 to the suction side of the compressor C. This cooling operation is conventional.

Heating operation of FIGS. 1-3

The dashed line arrows alongside the tubing of FIG. 1 show the direction of refrigerant flow during heating operation. When the thermostat HT calls for heating, it energizes the motor starters CMS and OFMS which close their switches CMSS and OFMSS respectively, starting the compressor motor CM and the fan motor OFM respectively; energizes through the normally closed switch DRS2 of the defrost relay DR, the reversal valve solenoid RVS which adjusts the reversal valve RV to its heating position, and energizes the rectifier bridge BR which supplies DC 4 through the busses 20 and 21 to the bridge containing the thermistors TH1 and TH2, and the resistors R1 and R2.

The compressor C supplies discharge gas through the tube 10, the reversal valve RV, and the tube 12 into the indoor coil IC operating as a condenser coil. Liquid condensed within the coil IC is expanded through the capillary tube 14 into the outdoor coil OC operating as an evaporator coil. Gas flows from the coil OC through the tube 11, the reversal valve RV and the tube 13 to the suction side of the compressor C. The heat pump is charged with sufficient refrigerant for cooling operation, and the restrictor 15 acts during heating operation to reduce the volume of refrigerant flowing into the outdoor coil OC operating as an evaporator as described in the previously mentioned patent of G. L. Biehn.

When low outdoor temperatures cause frost to form on the surface of the coil 00 while it is operating as an evaporator coil, the frost acts as insulation, reducing the heat absorbed from the outdoor air so that the temperature of the refrigerant within the coil OC decreases. The resulting decrease in the temperature of the thermistor TH2 causes its resistance to decrease without a corresponding decrease in the resistance of the thermistor TH1, thus causing an increase in positive voltage at the junction of the thermistors TH2 and TH1, unbalancing the bridge in which the thermistors are connected, and causing the amplifier A to energize the defrost relay DR. The latter opens its switches DRSl and DRS2, and closes its switch DRS3. The open switch DRSl deenergizes the fan motor starter OFMS which opens its switch OFMSS, stopping the outdoor fan motor OFM. The open switch DRS2 deenergizes the reversal valve solenoid RVS which adjusts the valve RV to its cooling position for operating the coil OC as a condenser coil to heat and melts the front on its surface. The closed switch DRS3 shorts out a portion of the resistor R2, decreasing its resistance, and causing an increase in the positive voltage at the junction of the thermistors TH1 and TH2, thus preventing a decrease in the positive voltage at the junction of the thermistors caused by the beginning of the melting of the frost, from deenergizing the relay DR, and maintaining the relay DR energized until all of the frost has melted from the surface of the coil OC. When the frost has melted, the temperature of the refrigerant Within the coil DC will have increased sufliciently to have caused the resistance of the thermistor TH2 to have increased sufliciently to cause the positive voltage at the junction of the thermistors to decrease sufficiently to cause the relay DR to become deenergized. The latter then reopens its switch DRS3, and recloses its switches DRSl and DRS2, returning the heat pump to heating operation.

Description of FIGS. 2d and 4 Referring to FIG. 2d, heat relay HR has a switch HRS which closes when the relay HR is energized.

FIG. 4 corresponds to FIG. 3, and difiers therefrom in that the thermistors are NTC thermistors; in that the defrost relay is not used, being replaced by simplier heat relay HR, and two transistors; in that the bridge RB is energized during cooling operation, and in that the resistor R2 is not tapped.

Referring now to FIG. 4, the compressor motor CM is connected by its starter switches CMSS to the lines L1 and L2. The fan motor OFM is connected by its starter switch OFMSS to the lines L1 and L2. The compressor motor starter CMS is connected in series with the cooling control thermostat CT, and in series with the heating control thermostat HT, to the lines L1 and L2. The heat relay HR is connected in series with tthe thermostat HT to the lines L1 and L2. The input to the bridge RB is connected in parallel with the starter CMS. The DC output of the bridge RB is connected to the busses 20 and 21. NTC thermistors TH1 and TH2 are connected in series, with the thermistor TH2 connected to the bus 20, and the thermistor TH1 connected to tthe bus 21. The

junction of the thermistors is connected to the base of NPN transistor TR1, the collector of which is connected by resistor R3 to the bus 20, and the emitter of which is connected to the junction of variable resistor R1 and resistor R2 connected in series to the busses 20 and 21. The collector of the transistor TR1 is also connected to the base of PNP transistor TR2, the emitter of which is connected to the bus 20, and the collector of which is connected in series with the outdoor fan motor starter OFMS to the bus 21. The reversal valve solenoid RVS is connected through switch HRS of the heat relay HR in parallel with the starter OFMS. The collector of the transistor TR2 is also connected in a positive feed back circuit through variable resistor R4 to the base of the transistor TR1.

Operation of FIGS. 2d and 4 When the thermostat CT calls for cooling, it energizes the compressor motor starter CMS and the rectifier bridge RB. The starter CMS closes its switches CMSS which energize the compressor motor CM. During normal operation, the resistor R1 is adjusted to cause the base of the transistor TR1 to be more positive than its emitter so that it is forward biased and conducts. The current drawn through its collector and the resistor R3 causes a voltage drop through the latter causing the base of the transistor TR2 to be more negative than its emitter so that it is forward-biased and conducts, and energizes the starter OFMS which closes its switch OFMSS, energizing the motor OFM of the outdoor fan OF. The heat pump then operates as described in the foregoing in connection with cooling operation.

When tthe thermostat HT calls for heating, it energizes the compressor motor starter CMS, the rectifier bridge BR, and the heat relay HR. The starter CMS closes its switches CMSS, energizing the compressor motor. The heat relay HR closes its switch HRS which connects the reversal valve solenoid RVS across the now energized starter OFMS, energizing the solenoid RVS which adjusts the reversal valve to its heating position. The heat pump then operates as described in the foregoing in connection with heating operation.

When frost forms on the surface of the outdoor coil 0C operating as an evaporator coil during heating operation, the temperature of the thermistor TH2 decreases, and its resistance increases. The voltage at the junction of the thermistors TH2 and TH1 decreases, and the base voltage of the transistor TR1 is lowered below its emitter voltage, so that the transistor TR1 is turned off. At the same time, the voltage at the collector of the transistor TR1 and the base of the transistor TR2 increases to that of the bus 20, and the transistor TR2 is turned oif. The solenoid RVS and the fan motor starter OFMS are deenergized. The latter opens its switch OFMS stopping the outdoor fan motor. The solenoid RVS adjusts the reversal valve to its cooling position so that the outdoor coil OC operates as a condenser coil to melt the frost on its surface. When the transistor TR2 is turned off, it acts as an open switch so that the resistor R4 in series with the parallel connected, relatively low resistance starter OFMS and solenoid RVS, is connected in parallel with the thermistor TH1, decreasing the resistance between the junction of the thermistors TH1 and TH2 and the negative bus 21 so that the voltage at the junction of the thermistors, and at the base of the transistor TR1 is so decreased that the transistor TR1 remains off after the start of the defrosting.

When the frost has melted from the surface of the coil DC, the temperature of the thermistor TH2 increases, and its resistance decreases, increasing the voltage at the junction of the thermistors, and at the base of the transistor TR1 so that the latter conduct and causes the transistor TR2 to conduct. The latter energizes the starter OFMS which closes its switch OFMSS which energizes the fan motor OFM. When the transistor TR2 conducts, it also energizes the solenoid RVS which adjusts the reversal valve to its heating position, returning the heat pump to heating operation.

Among the advantages of this invention are that the thermistors are cheaper, more reliable, and require less maintenance than the combinations of air pressurestats and thermostats previously used for starting and terminating defrosting.

I claim:

1. A heat pump comprising a refrigerant compressor; an outdoor coil; expansion means; an indoor coil; reversal means adjustable to a cooling position to route refrigerant from said compressor through said outdoor coil to operate the latter as a condenser coil, through said expansion means into said indoor coil to operate the latter as an evaporator coil, and from said indoor coil to said compressor, and adjustable to a heating position to route refrigerant from said compressor through said indoor coil to operate the latter as a condenser coil, through said expansion means into said outdoor coil to operate the latter as an evaprator coil, and from said outdoor coil to said compressor; and defrost means operable when said reversal means is in said heating position and said outdoor coil is operating as an evaporator coil; said defrost means comprising first and second thermistors connected in series, said first thermistor being exposed to the temperature of the outdoor air adjacent to said outdoor coil, said second thermistor being in heat exchange contact with said outdoor coil, means for flowing current through said thermistors, and means including means responsive to a change in voltage at the junction of said thermistors caused by frost forming on said outdoor coil and a resulting decrease in the temperature of said second thermistor, for adjusting said reversal means to said cooling position to operate said outdoor coil as a condenser coil.

2. A heat pump as claimed in claim 1 in which means is provided for causing an increase in said change in voltage when said reversal means is adjusted by said defrost means to said cooling position.

3. A heat pump as claimed in claim 2 in which said outdoor coil has a fan, in which there is an electric motor for driving said fan, in which there are electric supply connections and a switch for connecting said motor to said connections, and in which said defrost means includes means for opening said switch when said defrost means adjusts said reversal means to said cooling position.

4. A heat pump as claimed in claim 1 in which said outdoor coil has a fan, in which there is an electric motor for driving said fan, in which there are electric supply connections and a switch for connecting said motor to said connections, and in which said defrost means includes means for opening said switch when said defrost means adjusts said reversal means to said cooling posi tion.

5. A heat pump comprising a refrigerant compressor; an outdoor coil; expansion means; an indoor coil; reversal means; a solenoid for adjusting, when energized, said reversal means to its heating position to route refrigerant from said compressor through said indoor coil to operate the latter as a condenser coil, through said expansion means into said outdoor coil to operate the latter as an evaporator coil, and from said outdoor coil to said compressor, and for adjusting, when deenergized, said reversal means to its cooling position to route refrigerant from said compressor through said outdoor coil to operate the latter as a condenser coil, through said expansion means into said indoor coil to operate the latter as an evaporator coil, and from said indoor coil to said compressor; an electric driving motor for said compressor; a fan for said outdoor coil; a second electric motor for driving said fan; electric supply connections, a heating control thermostat: first and second normally closed switches: means, when said thermostat calls for heat, for connecting said compressor motor to said connections, for connecting through said first switch, said second motor to said connections, and for connecting through said second switch said solenoid to said connections; a cooling control thermostat; means when said cooling control thermostat calls for cooling, for connecting said compressor motor to said connections, and for connecting through said first switch, said second motor to said connections; and defrost means operable when said heating control thermostat is calling for heat; said defrost means comprising first and second thermistors connected in series, said first thermistor being exposed to the temperature of the outdoor air adjacent to said outdoor coil, said second thermistor being in heat exchange contact with said outdoor coil, means for flowing current through said thermistors, and means including means responsive to a change in voltage at the junction of said thermistors caused by frost forming on said outdoor coil and a resulting decrease in the temperature of said second thermistor, for opening said first and second switches.

6. A heat pump as claimed in claim 5 in which means is provided for causing an increase in said change in voltage When said switches are opened.

7. A heat pump as claimed in claim 6 in which said thermistors are connected with a pair of series-connected resistors in a bridge circuit, in which one of said resistors has a tap, in which a normally open switch is connected to said tap and to one end of said one resistor, and in which said means for causing an increase in said voltage change comprises means for closing said normally open switch when said first and second switches are opened.

References Cited UNITED STATES PATENTS 2,988,896 6/1961 Swart 62-160 3,113,439 12/1963 Eargle 62160 3,132,490 5/1964 Schmidt 62l60 WILLIAM J. WYE, Primary Examiner US. Cl. X.R. 62160 

